I turned up a 6over4 tunnel to Hurricane Electric with the IPv6 traffic from the tunnel passing through a Cisco ASA firewall. The stability and performance of the HE tunnel have been fantastic. Here is how it is setup:

While the Cisco ASA doesn’t support direct termination of IPv6 tunnels, it does have very rich support for IPv6 firewalling. The approach would be to take another external router which would sit outside of the ASA firewall and use that for tunnel termination. From there, bring the IPv6 inside interface of the router terminating the tunnel and connect that to an ASA interface which only has an IPv6 address.

You will need at least one additional static IP address on your outside network in order for this to work. Many Cable ISPs will offer this if you sign up for business level services usually for a nominal additional fee. It may still work with a dynamic address if it doesn't change all that much.

When you signup with HE they will provide two /64 networks and a /48. The first /64 IPv6 network is for the actual tunnel going from your external tunnel router to the HE tunnel server. The second /64 is for the connection between the inside interface of your tunnel router and the ASA. The final /48 is for internal subnets behind the ASA firewall.

As far as software goes, the ASA should be running software 8.2 or higher while the external IOS router could be running a recent image of 12.4T or Release 15. In my case I am using an 1800 series router running release 15.1.3T Advanced Enterprise.